I have terrible allergies that medication won't help. When someone has marinated in perfume on a plane, regardless of where they're sitting, I'll have a migraine for the rest of the day. On a flight back from London once, someone was actually using nail polish remover to give herself a manicure and then painting her toenails over. What kind of person does that on a crowded plane?
Another thing that makes me desperately ill is when people start lathering in anti-bacterial hand lotions and applying perfume and scented lotions. Everyone has a right to do their own thing, I admit, but when there's no air ventilation for several hours, please realize that not everyone on the plane wants to smell like you do.